As is known, rotors or fans for the first stage of large jet engines operate at speeds such that the tips of the blades thereof attain a velocity or "tip speed" in the supersonic range. Moreover, from these fans or rotors, when operational at supersonic velocities, there eminates a rotating field of sound pressure containing pure tone components of noise. Simply by eliminating these tonal components it is possible to reduce the noise level for jet aircraft by several decibels.
The invention generally relates to acoustic noise suppression devices for use in combination with air-breathing engines, and more particularly to a pure tone elimination assembly adapted to be mounted upstream from the air intake for a jet engine and employed for disrupting the continuity of forwardly projected, rotating fields of pure tonal sound pressure as they eminate from the engine.
2. Description of the Prior Art
During the course of a search conducted for the invention, hereinafter more fully described and claimed, the following patents were discovered:
______________________________________ 3,080,707 Vetter March 12, 1963 3,363,421 Ferri January 16, 1968 3,426,981 Allcock February 11, 1969 3,477,231 Paulson November 11, 1969 3,483,881 Pike et al December 16, 1969 3,511,337 Pease et al May 12, 1970 3,583,417 Clark June 8, 1971 3,610,262 Wise October 5, 1971 3,710,889 Lamy January 16, 1973 3,829,237 Chestnutt August 13, 1974 3,908,683 Demetrick September 30, 1975 ______________________________________
As is known, the noise produced by jet engines operating with supersonic relative tip speeds, in the fan or compressor first stage thereof includes multiple pure tone components forwardly propagated as a rotating field of sound pressure. Previously, such multiple pure tone noise has been attenuated with devices such as liners provided in the inlet. Such techniques, at best, tend to be partially effective and thus fail to meet existing needs.
Substantially all of the patents discovered during the course of the search contain disclosures of plates, vanes, or struts disposed in an intake flow path for a jet engine, excepting the patents to Pease et al and Ferri. However, in each instance, the purpose of the plates or vanes, as disclosed, is to "choke" or vary the velocity of the air flow for purposes of imparting supersonic speed to the flow for thus creating a sound barrier, particularly when the engine is operating at low power settings. This technique also fails to meet existing needs for various reasons, not the least of which are those which can be expected to attend the reduction in intake capacity for the engine. The patent to Pease et al, of course, discloses a noise attenuator for a gas turbine, wherein attenuation is effected by sound absorbing plates having off-set sections for deflecting the noise path, and the patent to Ferri discloses a ram-jet engine. The teachings of these patents are not deemed relevant. It is, therefore, believed that the prior art is devoid of any suggestion of the invention hereinafter more specifically described and claimed.
It is the general purpose of the instant invention to provide a practical, pure tone elimination assembly in combination with the upstream or air intake for a jet engine, having a bladed rotor characterized by tip speeds in the supersonic range.